connection with yolk sac retraction and differ- 

 entiation of the intestine. By various experi- 

 ments on the chick embryo, Mogg (1953; Moog 

 et al., 1955) has demonstrated that glucocorti- 

 coids, hormones secreted by the zona fasciculata 

 of the adrenal gland, accelerate the retraction of 

 the yolk sac in chicks and the later stages of dif- 

 ferentiation of the duodenal mucosa . She did not 

 propose a definite mode of action of the adrenal 

 cortical hormone, though she suggested an in- 

 direct effect on phosphatase activity and possibly 

 other enzymes. 



The various kinds of evidence seem to 

 indicate that DDT has a direct inhibitory and 

 destructive effect on the adi'enal cortical tissue 

 and that other effects are secondary. The 

 destruction of adrenal tissue of the fish exposed 

 to DDT and the atrophy of the zona fasciculata 

 found by others in animals exposed to various 

 chlorinated hydrocarbons might be considered 

 primary effects leading to the secondary effects 

 such as those found in the kidney, liver, and 

 intestine. The hyperexcitability and loss of 

 muscular control, the failure of trout yolk sac 

 resorption, the vacuolation in the liver, and the 

 deterioration of the intestine could be immediate 

 causes of death under extreme conditions . The 

 major characteristics for Addison's disease, a 

 hypoactive condition of the adrenal gland in man, 

 are similar for animals exposed to the chlorin- 

 ated hydrocarbons. Pigmentation is affected, 

 and there are gastro-intestinal disturbances, 

 weaker muscles, hypoglycemia, and some ef- 

 fects on reproduction. 



SUMMARY AND CONCLUSIONS 



Fourteen day bioassays were conducted 

 on young and adult guppies by exposing tiiem to 

 serial logarithmic dilutions of DDT under con- 

 trolled conditions. The values at which 50 per- 

 cent of the fish survived (TL/m) in 1, 4, 7 and 

 14 days were determined. On the basis of these 

 assays, it was determined that young guppies two 

 to three weeks of age are approximately 10 times 

 more sensitive to DDT than the adults. The 14- 

 day TL/m for the young was .0024 ppm and, 

 though the TL/m varied slightly with different 

 strains of fish, .0J8 ppm was established as 

 the sublethal dosage for the adult fish . 



Similar 14-day bioassays were conducted 

 with young brown trout . The fry, while still 

 depending on the yolk sac for food (two weeks old) 

 were found to be exceedingly resistant to DDT. 

 Over 50 percent were able to withstand . 18 ppm 

 DDT in a 14-day period. Once the yolk sac was 

 resorbed, the fish became very sensitive to the 

 insecticide. The 14- day TL/m for 10-week-old 

 fry was .00056 ppm and .014 ppm for the number 

 one fingerlings . 



The 14-day TL/m for adult guppies(.018 

 ppm DDT) was sublethal to adult guppies over an 

 extended exposure of 30 days. It was postulated 

 that exposed fish would not be harmed seriously 

 in DDT solutions of a concentration below the 14- 

 day TL/m . 



Limited observations on growth and repro- 

 duction indicated that exposure to DDT did not 

 prevent reproduction in guppies in the first 30 

 days of chronic exposure, but many of the guppies 

 born in DDT were dead or died within several 

 hours. The survival rate was lower for those 

 born in DDT than for those born in untreated 

 water. The data, however, were not sufficient 

 to make definite conclusions. Observations in 

 the literature indicate that after the first litter, 

 insecticides will inhibit reproduction. The cause 

 of the death of guppies in aquaria kept under con- 

 trolled conditions needs to be ascertained before 

 the effects of one dosage of DDT on reproduction 

 can be determined. 



Guppies born in DDT, especially in con- 

 centrations of .010 and .0185, had abnormally 

 large yolk sacs . The sacs were resorbed in 

 several days if the fry survived. Guppies born 

 in DDT and removed to untreated water were 

 slightly shorter after 40 days than guppies bom 

 in the control and treated similarly (averages 

 of 10 mm and 11 mm respectively), though the 

 data are inconclusive. 



Exposure of the guppies to sublethal 

 dosages of DDT for 14 days and then removal 

 to a concentration toxic to normal fish within 

 three days -- .032 ppm, demonstrated that the 

 fish had increased resistance to the toxicant 

 and the dosage in most cases was no longer 

 lethal. It was found that the greatest resistance 



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